Many kinds of data are transmitted over the Internet and other networks, including video and audio data. Data can for example be transmitted from one computer or other transmitting station to another remote computer or other receiving station. Data transmission over networks such as the Internet is frequently accomplished by packetizing the message to be transmitted—that is, by dividing the message into packets which are reassembled at the receiving end to reconstruct the original message.
Packets may be lost or delayed during transmission, resulting in corruption of the message. This can be especially problematic when it occurs during real time transmission of data (such as during voice over IP (VOIP) session or video conferencing). A video frame with lost or missing packets is referred to as an incomplete or partial frame.
Methods have been proposed to address the problem of packet loss. These methods include forward error correction (hereinafter “FEC” coding) and negative acknowledgment requests (hereinafter, “NACK” requests). FEC coding involves the transmitting station generating redundant FEC packets. These redundant FEC packets are transmitted along with the underlying source (i.e., video or other media) packets. The FEC packets can be used by a receiving station to reconstruct packets that are lost during transmission. Generally speaking, NACK requests are sent by the receiving station to request retransmission of lost packets that the receiving station cannot reconstruct using FEC packets. The receiving station can detect lost packets by identifying gaps in the packet sequence numbers of the packets that are received. The packet sequence numbers can be included in a packet header, such as an RTP header.